1. Field of Invention
This invention relates to a force measuring device incorporated into a towing ball-hitch or drawbar mechanism.
2. Background-Description of Prior Art
When automobiles, trucks or vans are engaged in towing a trailer unit, the most common type of connection used between the towing vehicle and trailer is a ball-hitch or ball-hitch and drawbar coupling. The ball-hitch fitting is attached to either a rear bumper fitting or a horizontally extending drawbar located on the rear of the so-equipped towing vehicle.
The typical ball-hitch is a unitary member having a spherical shape at the upper end and a threaded rod with a nut, or a bolt that threads into the lower end. The threaded nut or bolt is used to secure the ball hitch to the towing drawbar or bumper fitting of the towing vehicle. The drawbar on so-equipped vehicles is further inserted into a drawbar receiver and held in place by means of a retaining pin that may be rounded or rectangular in shape.
The ball-hitch fitting couples to a trailer ball-receiver or coupler fitting, located on the forward-most portion of the trailer's frame or "tongue" of the trailer. It is this "ball & socket" connection that allows for the trailer unit to rotate and pivot freely with respect to the ball-hitch and towing vehicle.
Ball-hitches are manufactured in a variety of industry-standardized sizes dictated by the load they are designed to tow, e.g., a two inch diameter ball-hitch typically has a load rating of 5,000 pounds, while a 2 & 5/16 inch diameter ball-hitch has a load rating of 7,000 pounds. The ball-hitch diameter must also correspond to the interior diameter of the "socket" or coupler receiver on the trailer's tongue fitting.
Once the ball-hitch has been coupled to the receiver or "socket" fitting located on the trailer, energies and forces are then transmitted between towing vehicle and trailer. For example, the static downward force of the trailer's tongue referred to as "tongue weight" would be transferred to the ball hitch and drawbar. Acceleration, deaccleration and lateral forces will also be transferred to the ball-hitch drawbar mechanism acceleration creating a pulling force between a towing vehicle and a towed vehicle and deceleration creating a compressive force between the vehicles.
Tongue weight is one important factor in keeping the coupler securely seated on the ball hitch and influencing whether the trailer will be towed in a balanced and stable manner. Improper tongue weight contributes significantly to unstable and unsafe towing characteristics in both the trailered unit and towing vehicle. For example, too much tongue weight will cause the towed vehicle to pitch downward excessively during deacceleration. Excessive tongue weight also contributes to excessive wear and tear on braking and transmission components. Too little tongue weight may lead to accidental separation of the ball hitch coupler connection and tends to induce pitch and yaw in the towed and towing vehicles. In addition, manufacturers of recreational passenger vehicles, trucks, vans and trailers will provide recommended tongue-weight ranges for their respective vehicles when engaged in towing. Tongue and force loads are the key elements a vehicle manufacturer is considering when determining the maximum towing capacity for any particular vehicle.
For background information relating to prior art pertaining to trailer hitch assemblies and particularly trailer ball hitch drawbar devices, reference may be made to the following U.S. Pat. Nos. 3,479,057; 4,201,400; 4,772,039; 4,864,874; 4,889,356; 5,040,817; 5,060,965; and 5,149,121.
U.S. Pat. No. 5,149,121 discloses a force measuring device wherein a trailer ball hitch component is immersed in an elastomeric material containing pressure sensing devices. A variation shows a modified ball hitch which by means of a 90 degree bend in the ball hitch fitting is attached to a vertical oriented sensing device.
The most obvious drawbacks to these designs are that the force loads and reactions of the trailer unit are transmitted to several non-standard components dramatically increasing the potential for structural failure and compromising the overall strength of the trailer ball hitch mechanism and coupling system. In addition, such previously disclosed devices require substantial modifications to existing standardized drawbar or bumper fittings. Many vehicles currently being manufactured could not be modified for installation of these designs. None afford the convenience of portability nor compatibility with existing industry standard bumper or drawbar receiver fittings. Security of the drawbar mechanism can be assured by the ability to quickly and conveniently remove it from the drawbar receiver. Less obvious shortfalls relate to accuracy and reliability of these designs since they do not provide for direct metal to metal contact between the sensing cell and the forces it is intended to measure. Immersing a load sensing cell in an elastomeric material introduces error factors caused by the static and temperature-altered physical properties of the elastomeric material itself. In addition, deterioration of the elastomeric material further limits the functional life of these designs.